In the treatment of patients it is often desirable to provide a means whereby a drug or like agent can be introduced in a controlled manner over an extensive period of time. With the conventional intake of drugs and the like by means of periodic ingestion of a capsule or tablet, or periodic injection, there is usually an initial significant increase in the amount of the active agent in the body which is then reduced over a period of time, until the next tablet is taken or injection given.
In many cases it is preferable to provide an intake of drug in a controlled manner such that the amount of drug in the body remains substantially constant. This is particularly the case in the treatment of patients suffering constant pain as in the case of terminally ill patients who require a constant amount of an analgesic to be administered in order for them to be able to withstand the pain.
In the past several devices have been proposed for controlling the introduction of the drug into a body whereby it can be delivered in a controlled manner over a period of time. Such devices have comprised pumps driven by a power source, which controls the delivery of drug to the body. Some of these pumps may be mounted externally to the body and are connected to a catheter introduced to a the body of the patient. Other devices have comprised a pump which is mounted subcutaneously to the body of the patient and which delivers a drug to the body at a desired location. Such devices suffer the disadvantage however they are quite expensive and that they utilise a power source in order to effect the delivery of the drug. In the event of the failure of the power source, the pump must be replaced. The power source has in the past taken a variety of forms including utilisation of a phase-change fluid as is the case of a device marketed under the trade mark “INFUSAID”.
Other delivery devices have comprised a manually operated pump which may be mounted externally to the body or subcutaneously in the body of the patient whereby the pump can be activated by the patient for the delivery of the drug as need arises for that drug. Examples of such devices are disclosed in U.S. Pat. Nos. 4,588,394, 4,681,560 and, 5,085,644 the contents of which are included herein by reference, and comprise devices whereby a pumping chamber is connected via catheter directly into the body and derives its source of drug from a holding reservoir. The pumping chambers are of the form whereby they are biased to a maximum volume condition, and on activation by the patient it is moved to a minimum volume position. In the case of U.S. Pat. No. 5,085,644 the re-entry of the drug into the pumping chamber is controlled by means of a capillary such that the pump is only able to deliver a full volume after a predetermined period of time. However, the disadvantage with this form of delivery means is that drug levels within the body will vary periodically whereby on activation of the pump, the drug level within the body initially has a high level which is then allowed to decay over a period of time until the pump is reactivated.